A student makes the following argument: 'The defects of vision — myopia, hypermetropia, and presbyopia — are all essentially the same problem: the image does not fall on the retina. So they should all be correctable with the same type of lens.' Evaluate this argument. Identify what is correct in it and where the reasoning breaks down, referring to the causes and corrections of each defect.
Generated by claude-sonnet-4-6 · 2026-06-26 01:09 · grounding rag
Model Answer
What is correct: The student is right that in all three defects, the image does not fall on the retina, causing blurred vision. It is also true that all three are correctable using spherical lenses.
Where the reasoning breaks down: The three defects differ in cause and direction of image shift, so they require different types of lenses:
- Myopia: Image forms in front of the retina due to excessive curvature of the eye lens or elongation of the eyeball. Corrected by a concave (diverging) lens.
- Hypermetropia: Image forms behind the retina because the focal length of the eye lens is too long or the eyeball is too small. Corrected by a convex (converging) lens.
- Presbyopia: Caused by weakening of ciliary muscles and reduced flexibility of the eye lens with age, reducing the power of accommodation. Often requires bi-focal lenses (concave portion for distant vision, convex for near vision).
Since the image displacement and underlying causes differ, one lens type cannot correct all three defects.
Source: Chapter 10, Section 10.2 – Defects of Vision and their Correction
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Explanation
- Examiners expect you to acknowledge the valid part of the argument first — it shows analytical thinking.
- The key error to expose is that image displacement direction differs (in front vs. behind retina), demanding opposite lens types.
- Presbyopia is distinct — it's about loss of accommodation, not just image position — so it gets special mention (bifocals).
- Name each defect, its cause, image position, and correction lens. That covers all 5 marks.